In improving fuel efficiency and increasing outputs of an internal combustion engine, to reduce friction at sliding portions in its valve train assembly is an effective measure. Therefore, for the purpose of reducing friction at the sliding portion between a valve lifter and a cam, the sliding surface has been coated with a hard film, such as titanium nitride films (e.g., TiN), chromium nitride films (e.g., Cr2N, CrN), or diamond-like carbon (DLC) films.
To reduce friction, it is necessary that the surface roughness of a shim that is located on the top surface of a valve lifter and slides against a cam, should be as small as the thickness of oil film between the cam and the shim or top surface of valve lifter. For example, Japanese Laid-Open JP05-163909A, which is incorporated herein by reference, discloses a structure of cam contact part, in which the surface roughness of a shim is made to have a ten-point mean roughness (Rz) equal to or less than 0.1 micrometers (which corresponds to 0.025 micrometers in terms of Ra), and the surface of the shim (or top surface of valve lifter) base material is coated with a thin film such as TiN, TiC, TiCN, CrN, or DLC. With this structure, the contact surface on the side of the cam that contacts with the shim is smoothed to a mirror-like surface during initial sliding operation. In another example, JP2002-309912A, which is incorporated herein by reference discloses a combination of a shim, a lifter, and a camshaft being excellent in friction characteristics and durability, which prevents cracking and flaking of hard thin films that are inherently less ductile and thus, realizes reliability in durability and low friction coefficient. This is provided by making the surface roughness of a base material, before forming a hard carbon film, such as a DLC film, to be equal to or less than Ra 0.03 micrometers, and by coating the base material with a film whose surface roughness is specified depending on hardness and film thickness.
Among these hard films for coating the surface of sliding portions, DLC films have been studied for practical use because of their high hardness and low friction coefficient, expecting their wear resistant properties and friction reduction in a direct acting valve train system. DLC films have a plurality of types depending on, for example, composition ratio of diamond structure to graphite structure, or content of hydrogen or metal. However, in general, a DLC film means an amorphous hard carbon film. Known methods for forming an amorphous hard carbon film include a CVD (chemical vapor deposition) method, using hydrocarbon gases such as methane or acetylene, and a PVD (physical vapor deposition) method, using graphite or the like as a target.
An amorphous hard carbon film formed by a CVD method contains more hydrogen than the film formed by a PVD method, and thus it is difficult to obtain sufficient adhesion to a ferrous base material due to internal residual stress. Therefore, to obtain better adhesion to the base material, methods for reducing internal stress have been employed by interposing an interlayer consisting of metal or metal and carbide, or by incorporating metal into the amorphous hard carbon film.
On the other hand, an amorphous hard carbon film formed by a PVD method such as a Vacuum Arc Ion Plating method has a characteristic of being substantially hydrogen-free, except for the hydrogen unavoidably contained in general (on the order of a few atomic %). Such a film is considered to have a higher hardness and more excellent wear resistance than the amorphous hard carbon film formed by a CVD method. According to JP2002-309912A and JP2004-137535A, which is incorporated herein by reference, specific characteristics of hydrogen-free amorphous hard carbon films are as follows. Film thickness are 0.33 to 1.90 micrometers, Knoop hardness of the surfaces are 1956 to 4050, and hydrogen content are equal to or less than 0.5 atomic %; and at that time, the hardness of the base materials are HRC 53 to 60 and the surface roughness of the base materials are Ra 0.01 to 0.03 micrometers. In addition, also disclosed is the fact that it is desirable that the surface roughness of the base material be as small as possible, and that, if the surface roughness of the base material is smoothed to on the order of Ra 0.01 micrometers, the surface roughness of the film, after the film is formed, can be on the order of Ra 0.03 micrometers without finishing process.
In JP05-163909A, which is incorporated herein by reference, the sliding portions has a structure, in which a base material whose surface roughness before forming a hard film is made to be equal to or less than Rz 0.1 micrometers and a thin hard film is coated on the base material. However, with this structure, by the time the opposing material is smoothed to be mirror-smooth by sliding, the hard film is ground away. Thus, there is a problem that wear occurs on the sliding surface of the base material after the hard film is ground away.
In JP2002-309912A, which is incorporated herein by reference, the surface roughness of a base material before forming a hard carbon film is made to be equal to or less than Ra 0.03 micrometers. On the base material, a hard thin film whose surface roughness Ry is specified depending on hardness and film thickness, is coated by an Arc Ion Plating method to prevent cracking or flaking. However, when grinding, using a general grindstone or lapping or buffing using liberated grains, is performed on the surface of the base material before forming a hard film, a multiple of unavoidable and continuously extending scratches remain on the surface of the base material due to hard particles of abrasives. Therefore, an amorphous hard carbon film coated on the surface of such a base material suffers from a problem of occurrence or development of flaking that is originated from the scratches on the surface before forming the hard film, even though adhesion to some extent is obtained by metal ion bombardment.